The correct answer is b) liquid.
Liquids are unique because they have a definite volume, meaning they will not compress to occupy less space. However, unlike solids, they do not have a fixed shape. Instead, liquids conform to the shape of their container. This behavior is due to the molecular structure of liquids, where the molecules are close together but can move past one another, allowing the liquid to flow and fill the bottom of any container it is in.
In contrast, solids have both a definite volume and a fixed shape. Gases, on the other hand, do not have a definite volume or shape; they expand to fill their entire container. Therefore, the answer is liquid as it fits the description provided in the question.